A switched reluctance machine (SRM) is well known in literature and its principle, theory of operation, and construction are described in R. Krishnan, “Switched Reluctance Motor Drives”, CRC Press, 2001. An SRM has windings on its stator poles and no windings or magnets on the rotor poles or rotor slots. SRMs are ideal for high torque, are highly fault-tolerant, are highly efficient, and have high thermal operating conditions. Recently, machines with SRM e-core structures (see Cheewoo Lee, R. Krishnan, and N. S. Lobo, “Novel Two-Phase Switched Reluctance Machine Using Common-Pole E-Core Structure: Concept, Analysis, and Experimental Verification,” IEEE Trans. Ind. Appl., vol. 45, no. 2, pp. 703-711, March-April 2009) and SRMs with unexcited common poles (see C. Lee and R. Krishnan, “New designs of a two-phase e-core switched reluctance machine by optimizing the magnetic structure for a specific application: Concept, design, and analysis,” IEEE Transactions on Industry Applications, vol. 45, no. 3, pp. 1804-1814, September-October 2009) have been introduced with high power density and high efficiency operation. In the SRM e-core structure and SRM with unexcited common poles, maximum efficiency has been extracted from an electromagnetic point of view. To increase torque density of an SRM further, a structural change in its stator or rotor cores or windings is not sufficient; other means must be used to obtain much higher power density and greater efficiency. One way to achieve higher power density is to focus on the excitation augmentation in a stator with permanent magnets (PMs). Such augmentation of the excitation must bestow the fundamental operational characteristics of the SRM so as to maintain the attractive features of: (1) dc excitation; (2) simplicity, with regard to the minimum number of power semiconductor devices employed to control current in a power electronic circuit; (3) absence of shoot-through faults; (4) high fault tolerance, (5) utilization of reluctance torque; and (6) high efficiency operation (see see R. Krishnan, “Switched reluctance motor drives”, CRC Press, 2001).
Many machine implementations have been in practice, such as one with PMs in a stator back iron (see R. Krishnan, “Permanent magnet synchronous and brushless dc motor drives”, CRC Press, 2009 and X. Luo, and T. A. Lipo, “Synchronous/permanent magnet hybrid AC machine,” IEEE Transactions on Energy Conversion, vol. 15, no. 2, pp. 203-210, 2000) to exploit the structural properties of an SRM. Placing PMs in a stator back iron creates operation that is not that of a switched reluctance machine drive system, but is that of a PM brushless direct current (dc) motor drive system, which causes the loss of the best operational features of SRMs while embracing only the standard features of a PM brushless dc motor (PMBDC) drive system. Different schemes for realizing an SRM with PMs in the stator are provided in detail in R. Krishnan, “Permanent magnet synchronous and brushless dc motor drives”, CRC Press, 2009, and briefly described here. Broadly, three kinds of PMs superimposed on SRM structures can be seen in the literature; they are: (1) PMs in a back iron of a stator, (2) PMs on stator pole faces, and (3) PMs embedded in the middle of stator poles.
FIG. 1 illustrates a machine 100 with PMs 101 and 102 disposed within a stator back iron 104. PMs 101, 102 aid the flux arising from stator pole excitations. An SRM of this type is known in literature as a PM SRM or doubly-salient PM machine. The reversal of excitation currents in stator pole windings 103 will produce flux with polarity that is opposite to the flux from PMs 101, 102. The opposite polarities of flux causes flux cancellation, due to opposing magneto-motive force. Flux produced by the stator current excitation passes through adjacent stator poles, resulting in greater flux leakage. The reluctance variation for the phase becomes smaller leading to smaller reluctance torque. Therefore, the main torque produced is primarily the synchronous torque of the machine. The doubly-salient PM machine behaves like a PM synchronous machine or, what is sometimes referred to in the literature, a PM brushless dc machine. PMs 101, 102 equivalently replace PMs in a rotor of a conventional PMBDC machine, with no apparent difference in performance.
FIG. 2 illustrates a single-phase machine 200 having two PMs disposed on the face of each stator pole. FIG. 3 illustrates a three-phase machine 300 with PMs disposed on the pole faces. The structures of FIGS. 2 and 3 support flux reversal in the back iron 205 of machines 200, 300 (see R. P. Deodhar, S. Andersson, L. Boldea et al., “Flux-reversal machine: A new brushless doubly-salient permanent-magnet machine,” IEEE Transactions on Industry Applications, vol. 33, no. 4, pp. 925-934, 1997). Machines 200, 300 are referred to as flux reversal machines.
In machines 200 and 300, PMs 201 and 202 are installed on the face of each stator pole 203. PMs 201, 202 are magnetized along the radial direction of machine 200's and machine 300's rotors 204. Each stator pole 203 has two magnets 201, 202 that are half the width of the pole arc. Net flux in each stator pole 203 is the sum of the flux due to the windings and the flux of the PMs. Only half of the stator pole face is utilized by the flux when stator windings 206 are excited. The wide face of each stator pole, as illustrated in FIGS. 2 and 3, increases the difficulty of manually wrapping windings on the poles and automated winding insertion is expensive.
FIG. 4 illustrates a three-phase machine 400 having PMs embedded in the middle of each stator pole (see D. Ishak, Z. Q. Zhu, and D. Howe, “Comparative study of permanent magnet brushless motors with all teeth and alternative teeth windings,” IEE Conference Publication, pp. 834-839, 2004). PMs 401 are embedded in the center of poles 402 along the radial direction of rotor 403. The magnetization of each PM 401 is along the circumferential direction of machine 400. The performance of machine 400 is similar to that for flux reversal machines 200, 300. Machine 400 is referred to as flux switching machine (FSM) in literature. The operation of an FSM is the same as that of a PM synchronous machine, and the stator windings are excited with three-phase alternating currents. FSMs have no reluctance torque and, thus, no relationship to SRM operation or characteristics.
In summary, doubly-salient PM machines, flux reversal machines, and FRMs: (i) are fundamentally alternating current (ac) machines, (ii) have PMs embedded in the stator instead of in the rotor, as is conventional, (iii) allow flux reversal in the stator poles to varying degrees, and (iv) have an SRM structure of salient stator and rotor poles, but function solely as PM synchronous or brushless dc machines, as they have very negligible reluctance torques.
FIG. 5 illustrates an SRM 500 having four excitation poles 501, 502, 503, and 504, two poles for each of two phases. Phase A employs excitation poles 501, 502, and Phase B employs excitation poles 503, 504. SRM 500 has windings 505 on each of excitation stator poles 501, 502, 503, 504. Diametrically opposite excitation pole windings constitute a phase winding. SRM 500 has a common pole 506 sandwiched between each pair of adjacent excitation poles 501, 502, 503, 504. Thus, SRM 500 has four common poles 506 and four excitation poles 501-504. When phase A is excited, flux: (1) enters excitation pole 501; (2) is conveyed to back iron 507; (3) passes through each common pole 506 adjoining excitation pole 501, (4) enters the air gap (i.e., the space existing between a rotor pole and its nearest stator or common pole at any instant) between the common pole conveying the flux and its nearest rotor pole 508; (5) enters the nearest rotor pole 508; (6) enters rotor back iron 509; (7) enters a rotor pole nearest excitation pole 501; (8) enters the air gap between excitation pole 501 and its nearest rotor pole; and (9) is conveyed back to excitation pole 501. The main flux in excitation pole 501 splits between left- and right-side common poles 506 and likewise for excitation pole 502 of phase A.
The purpose of each common pole 506 is to carry the flux produced by the individual excitation pole nearest to the common pole and route it back to the excitation pole via the air gap, rotor pole, adjoining rotor back iron, rotor pole, and air gap. The same applies to phase B operation in the SRM. From the above-discussion, it may be inferred that the flux in the common poles does not reverse, regardless of which phase is conducting. The excitation poles experience no reversal of flux, as they are excited only with unidirectional current. Common poles 506 serve to carry the flux generated by excitation poles of both phases.
Flux generated in machine 500 is solely due to the excitation of phase windings 505 on excitation poles 501-504. A challenge facing machine 500 is that the starting torque at an unaligned position of the rotor and stator poles is not very high.